Photodiodes are devices designed and used to sense and detect light at various wavelengths (or energies) and intensities by converting incident radiant power into electrical power, where the intensity of the produced electrical signal is proportional to the power of the incident light. These devices are typically fabricated using semiconductor materials, such as silicon (Si), germanium (Ge), indium (In), gallium arsenide (GaAs), or combinations thereof, as the active, light-sensitive component. The key component of the device is a p/n junction, which is an interface between the positively doped p-region and negatively doped n-region of a semiconductor. The doping is achieved by incorporation of an element with a deficiency (p) or excess (n) of electrons into the semiconductor.
The principle of operation of a photodiode is based on the photoelectric effect. When a photon of sufficient energy strikes the photodiode, it creates an electron-hole pair. If the electron and “hole” are generated near the p/n junction, they are swept from the junction by the internal electric field. The holes are collected at the anode, and the electrons are collected at the cathode (typically with the help of an applied external electric field), producing photocurrent with magnitude proportional to the number of absorbed photons. Commercially available photodiodes have number of limitations, such as limited sensitivity (or spectral responsivity), low temperature stability, low resistance to ionizing radiation, etc. Accordingly, an improved photodiode may be beneficial.
Nuclear batteries are devices designed and used to convert the energy of a decaying radioactive material into electrical energy. The primary application of nuclear batteries is their use as a long-lasting energy source, where the lifetime of the battery is related to the half-life of the radioactive material. These half-lives can be as long as years or centuries. The conversion of the radiated isotope energy into electrical energy in a nuclear battery can be based on several effects/principles, such as: alpha-voltaic, beta-voltaic, gamma-voltaic, scintillation intermediate, thermoelectric, thermophotoelectric, direct charge collection, and thermionic.
In case of alpha-voltaics, beta-voltaics, or gamma voltaics, for example, a, (3, or γ radiation produced by the radioactive isotope is captured and directly converted into electrons and holes in a semiconductor material. The separation of the generated electrons and holes is achieved by incorporation of a p/n junction into a semiconductor material. In case of scintillation intermediate, α, β, or γ radiation produced by the radioactive isotope is first captured by a liquid or solid scintillator material that absorbs the ionizing radiation and remits the radiation in the form of ultraviolet (UV), visible, or near-infrared photons. These photons are then absorbed by an adjacent semiconductor material and converted into electricity via a photovoltaic effect in the same fashion as in a traditional solar cell. Commercially available silicon solar cells have been explored in this application.
In a thermoelectric version of a nuclear battery, the heat generated by the decay of a radioisotope is converted into electricity through the Seebeck effect. In principle, a nuclear battery can generate electricity through one or combination of these effects.
A common feature of all nuclear battery designs is that they use a semiconductor material to convert the energy of the radioisotope into the electrical energy. This design requirement is also a source of limitation for the nuclear batteries as the semiconductors degrade upon exposure to the ionizing radiation. Ionizing radiation displaces atoms within the crystal lattice of the semiconductor, thus creating a vacancy. The rate of displacement is directly proportional to the power density of the radiation produced by the source material. Thus, the high power density of the ionizing material necessary for efficient generation of electricity is typically responsible for rapid degradation of performance of nuclear batteries. Accordingly, an improved nuclear battery may be beneficial.